Magnetorheological (MR) fluids belong to a class of controllable fluids. The essential characteristic of these fluids is their ability to change from a free-flowing, linear, viscous liquid to a semi-solid with controllable yield strength in milliseconds when exposed to a magnetic field. In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian fluids.
Magnetorheological energy absorption (MREA) devices harness the ability of MR fluids to change yield strength with a change in applied field. MREA devices are referred to as “tunable”, meaning that the resultant yield strength, and therefore energy absorption capability, can be varied by controlling the applied magnetic field. MREA devices have been identified as candidates for tunable impact energy absorption applications, meaning those in which a high shock load is applied during a short time period. Heretofore, MREA devices have been less than ideal for many automotive applications related to impact energy management and control of deceleration because of their large size and the lack of significant field controlled tunability of their stroking force (i.e., damping force) over the required range of stroking velocities. Tunability of damping force is critical to the desirability and usefulness of MREA devices in many applications, such as automotive applications where control of deceleration is important. For example, a damping force suitable for absorbing energy in one impact event may be too large for another, in which case tunability of the MREA device to respond with a lower damping force, and therefore a lower deceleration, is desirable.